The DC7 is the 7th in a serie of home made transmitters/ transceivers I have built over the years : it is a high-performance H.F transceiver, at least as far as the receiver is concerned.
The receiver is a single conversion one using an 8 MHz IF : I have
chosen this approach while many commercial transceivers feature
multiple conversions because of less potential birdies generation and
also because the DC7 is a ham band only equipment.
After narrow bandpass filtering, the signal is possibly amplified by a switchable RF amplifier feeding a high dynamic range mixer built around an FST3125 IC. This mixer was presented some years ago by C. Horrabin G3BSI and tested by many other hams : its input IP3 is around +40 dBm, a noteworthy and uncommon value for ham equipment. A large signal post mixer amplifier, fed by the output of the FST3125 is connected to the input of the IF filters.
Two bandwidths are available, 2300 and 400 Hz provided by two home made crystal ladder filters (Click here for more information) . The SSB filter is a 12 poles Chebycheff design while the CW is a 6-pole Butterworth one.
The IF amplifier and AGC uses 4 Analog Devices AD603 ICs and incorporate ideas proposed by several authors (B. Carver K6OLG, W. Hayward W7ZOI, M. Mandelkern KN5S and others).
A 4 poles ladder filter, located near the end of the IF chain, is used as a noise filter.
The product detector is a classic Double Balanced Mixer driven by a BFO whose frequency can be voltage controlled. It is followed by a low pass, low noise Chebycheff audio filter and the audio amplifier.
In order to minimize the local oscillator phase noise, the DC7 presently uses a standard low drift VFO but I'm thinking about a quiet DDS synthesizer.
The DSB signal generated by a NE602 balanced modulator is
connected to the SSB 12 poles filter followed by the transmit mixer.
The PA uses a push pull of 2SC1969 in a circuit inspired by the
Elecraft K2 one. Low pass filters are connected between the PA output
and the antenna connector.
Main characteristics :
|Minimum Discernable Signal (BW=2.3kHz)||-134 dBm|
|Blocking Dynamic Range||130 dB|
|Two Tones Third Order IMD (20 kHz spacing)
|Third Order Input Intercept Point||+20 dBm|
|IF AGC Dynamic Range||>100 dB|
|Power Output||10 W|
|Carrier Rejection||>50 dB|
|Undesired Sideband Rejection||>60 dB|
|Third Order IMD||>35 dB|
First works on the DC7 began many many years ago and its design has been a long story. From time to time, I still try to improve some circuits and publish the results on these pages.